As I understand it, the popular “self destruction” story of the people of Rapa Nui doesn’t hold much water, at least on its own. Rather, later events such as abduction by slavers, infectious diseases and “cultural extermination” by missionaries lead to cultural decline/change and loss of knowledge. Of course, all cultures change and knowledge is lost over time but, as I understand it, no one can even decipher the written language (whether it is proto- or true writing) and it’s not that long ago that people presumably could.
While more can always be said, there have been a few good answers on this previously:
First, an extensive overview of the society by /u/mikedash
Second, a criminally overlooked one by /u/legendarytubahero on the population and literacy crash
And third, /u/mikedash and my own comments in this thread, which I'll repost part of as a summary:
Of the 1500 or so people who were deported to the Peruvian mines and such [in 1862], that cadre included almost all of the two classes that were literate: the royalty and priest castes of the island. The couple dozen or so survivors who were returned later also brought smallpox back, and killed off most of the rest of the population and by most accounts finished the job.
There's evidence that there were once far more rongorongo than the 26 that survived, but with literacy completely wiped out and wood short, most of them were apparently repurposed around that time for more basic necessities like constructing boats for fishing. Thus, while I've periodically reviewed what's new in the lit since that trip and seen some genuinely creative work chasing archeological leads that is usually way over my quant social scientist head, the fact that we have basically zero written or oral history left - and almost certainly never will - is something that is important to keep in mind when viewing conclusions here.